In the present design of most wood golf clubs, excessive side spin is generated to the ball on all off-center hits on the clubface. This characteristic is usually called gear-effect and is inherent in wood golf clubs, in which the lead swingweight is installed too far from the face of the clubhead and without due consideration for good weight distribution in the clubhead assembly.
The lead swingweight usually is installed in a round cavity, drilled in the bottom or sole of the wood clubhead, peened and expanded to secure it in place, and then covered by a metal sole plate.
Considering the high impact forces, lead does not have the structural properties desired for this application and may be an energy absorbing component of the clubhead.
Bulge radius is used on the face of wood clubheads to send the golf ball further off the target line to correct for the excessive part of the gear-effect curve, and the ball will land on the fairway but with some loss in distance.
Improper weight distribution and incorrect center of gravity location in the wood clubhead assembly require the use of bulge radius to correct for these deficiencies in the wood clubhead design.